Photomultiplier or image amplifier with secondary emission transmission type dynodes made of semiconductive material with low work function material disposed thereon



Nov. 11, 1969 I O ET AL 3,478,213

PHOTOMULTIPLIER OR IMAGE AMPLIFIER WITH SECONDARY EMISSION TRANSMISSIONTYPE DYNODES MADE OF SEMICONDUCTIVE MATERIAL WITH LOW WORK FUNCTIONMATERIAL DISPOSED THEREON Filed Sept. 5, 1967 2 Sheets-Sheet 1 BYMWArman Nov. 11, 1969 R, sl o ETAL 3,478,213

PHOTOMULTIPLIER OR IMAGE AMPLIFIER WITH SECONDARY EMISSION TRANSMISSIONTYPE DYNODES MADE OF SEMICONDUGTIVE MATERIAL WITH LOW WORK FUNCTIONMATERIAL DISPOSED THEREON Filed Sept. 5, 1967 2 Sheets-Sheet 2 [kw/via:514; .5 674mm, ikomv/i'W/uuw 5 F40 Mxmwa/ A T TORUEY States atentPHOTOMULTIPLIER on IMAGE AMPLIFIER-p ABSTRACT OF THE DISCLOSURE I Asecondary emission device of the transmission type TRANS- which may beused as a photomultiplier, lightamplifier, or X-ray image intensifier.The device achieve amplifi cation of an electron image by causingtheelectrons to impinge upon a thin film of semiconductor material. athigh velocity, thus generating a large number, of secondary electronsfor each incident (primary) electron. The .secondary electrons diffusethrough the semiconductive film and are emitted from the oppositesurface of the film which is coated with a monomolecular layer of cesiumto reduce the work function at the emitting surface.

The secondary emission semiconductive'film consists of highly doped Ptype gallium phosphide. The energy levels in the film structure are suchthat electrons in the conduction band can reach the emitting surfacewith a residual energy above that of the vacuum energy level, so thatelectrons are emitted from the cesium-coated surface without thenecessity of supplying additional energy.

Background of the invention This invention relates to the field ofsecondary emission transmission type devices, and more particularly toan improved secondary emission device utilizing a dynode which includessemiconductor material.

The secondary emission electron tube devices which are presently incommon use are of the reflecting type; i.e., they operate in a mannerwherein the secondaryelectrons are emitted from the same surface as thatupon which the primary electrons to be amplified impinge. Thetransmission type of secondary emission electron amplifying structure iswell known in the art, but has not achieved commercial acceptability dueto a number of inherent disadvantages of the transmission structure indevices heretofore known. I

Basically, the transmission type of secondary emission device utilizes adynode element in the form of a very thin (on the order of a few hundredAngstroms in devices heretofore known) composite film, such that whenincident primary electrons bombard one major surface of the film,secondary electrons are emitted from the opposite major surface of thefilm. Such a device is exemplified, e.g., by US. Patent No. 2,898,499.

Since the secondary emission materials employed in such prior artdevices rapidly attenuate any movement of conduction electrons in thebulk material, it has heretofore been necessary to make the dynode filmsextremely thin, and consequently mechanically fragile. When thesecondary emission layer is made very thin, it does not absorb all ofthe incident (relatively high energy) primary electrons. As a result,some of the primary electrons pass completely through the dynode filmand impinge upon the phosphor screen or other detection means Since thevery thin dynode structures previously employed do not absorb allincident primary electrons, they necessarily, operate at reducedefficiency. Thevery thin dynode film, being somewhat transparent tovisible light, "permits feedback from the phosphor screen (when the -thematerials heretofore available for such devices exhibit relatively lowsecondary emission yields (less than 10 secondary electrons for eachprimary electron) so that a great many cascaded stages are required toachieve useful values of electron image amplification.

An objectof the present invention is to provide an improved [secondaryemission transmission type device haying a dynode structure ofrelatively great thickness compared to those heretofore known, andexhibiting a secondaryemission yield and signal to noise ratiosubstantially greater than that of prior art devices of this typfi;

. Summary According to the invention there is provided a transmissiontype secondary emission device including (i) a source of primaryelectrons which are made to impinge upon one major surface of asecondary emission dynode of semiconductive material to producesecondary electrons which are emitted from the opposite major surface ofthe dynode, and (ii) means for utilizing the emit-ted secondaryelectrons, in which the dynode comprises a highly-doped film of P typesemiconductive material and a thin layer of an electropositive workfunction reducing material on the surface of the semiconductor fromwhich the secondary electrons are emitted; the energy gap of thesemiconducting material, the work function at the surface of theelectropositive layer, and the acceptor impurity concentration in thesemiconductor bulk are such that the bottom of the conduction band inthe semiconductor bulk lies at an energy level above the vacuum energylevel at the exposed surface of the electropositive layer; the thicknessof the semiconductive film is chosen sufficiently great so as to absorbthe incident primary electrons, but not greater than a value. on theorder of a few times the diffusion length for electrons in thesemiconductor hulk, so that the secondary electrons may diffuse towardand be emitted from the exposed surface of the electropositive layer.

In the drawing the dynode of FIGURE 2 taken along the cutting planeutilized in the device so ayto greatly reduce 'the effective I imagecontrast obtainable.

A-A' and FIGURE 3B shows an energy level diagram useful in explainingvarious features of the invention.

Detailed description FIGURE 1 shows an electron multiplier device 1constructed according to the invention. The device 1 is in the form of acylinder having an opaque outer insulating shell 2 which may comprise asuitable ceramic material. At opposite ends of the cylinder aretransparent end plates 3 and 4 which may comprise a suitable glass.

The device 1 may be employed as a light image amplifying device, or as asimple photomultiplier. Incident radiation, which may comprise visible,infrared or ultraviolet light or X-rays, passes through the end plate 3(which must, of course, be designed to transmit'the particular type ofincident radiation to be detected) and through the transparentconductive film 5 to impinge upon the photoemissive layer 6.

Where the incident radiation is a visible light image, the conductivelayer 5 may comprise a material such as stannic oxide, which issubstantially transparent to visible light. The photoemissive layer 6may then comprise one of the well known materials which emits electronswhen subjected to irradiation by visible light, such as cesium antimony.Electrons are emitted from the exposed surface 7 of the photocathode 6in accordance with the incident radiation, to form an electron imagewhich corresponds to the visible light image to be amplified. Theemitted electrons are accelerated by the voltage V across resistor R(which is supplied by a voltage divider network consisting of voltagesource E and resistor R through R to impinge upon the surface 8 of thesecondary emission dynode 9. These incident primary electrons, whichhave a relatively high kinetic energy due to the potential differencebetween the dynode 9 and the transparent conductive layer 5, areabsorbed by the semiconductive film 10 of the dynode 9, so that eachincident primary electron produces a relatively large number ofsecondary electrons in the bulk of the semiconductive film 10. Arelatively large proportion of these secondary electrons diffuse towardthe low work function layer 11 and are emitted from the exposed surface12 of the layer 11.

The improved efiiciency", i.e. the ratio of the number of secondaryelectrons emitted from the layer 11 to the total number of secondaryelectrons created in the bulk of the film 10, of our dynode structure isattributable to the fact that conduction band electrons having thermalenergies can travel relatively large distances in the bulk of the film10 and be emitted from the exposed surface of the layer 11. i

Due to the improved efiiciency and higher gain of our dynode structure,random fluctuations of the number of emitted secondary electrons(corresponding to an invariant stream of incident primary electrons) areminimized, thus resulting in an improved signal to noise ratio.

The secondary electrons emitted from the surface 12 of the dynode 9 areaccelerated by the potential difference V across resistors R and R toimpinge upon the surface 8' of the next dynode 9', so that theaforementioned process is again repeated, leading to the emission ofadditional secondary electrons from the exposed surface 12' of the lowwork function layer 11'. The process then proceeds in similar fashion,with the electrons emitted from the surface 12 being accelerated by thepotential difference V across resistors R and R to impinge upon thesurface 8 of the dynode 9", so as to cause additional secondaryelectrons to be emitted from the exposed surface 12" of the low workfunction layer 11".

The resultant amplified electron image, in the form of secondaryelectrons leaving the surface 12", is converted into visible light bycausing the electrons to impinge upon a phosphor screen 13 which maycomprise an electronsensitive light-emitting material such as zincsulphide. The electrons emitted from the surface 12 are acceleratedtoward the phosphor screen 13 by means of a potential difference Vapplied between the dynode 9" and a thin electron-permeable aluminumlayer 14 applied to the surface of the phosphor layer 13.

Typically, the source E may have a voltage on the order of 10 kilovolts,while the voltages V V and V may have values on the order of 2kilovolts, the phosphor screen accelerating voltage V; being on theorder of 4 kilovolts. These values will, of course, depend upon thedesired parameters of the specific device to be constructed.

In some cases, it may be desirable to modulate or more uniformly controlthe acceleration of electrons between the various dynodes by means ofgrids 15 and 16. Where necessary or desirable, the electrons travelingthrough the device 1 may be focused by means of a magnetic fielddirected parallel to the axis of the cylinder formed by the insulatingshell 2.

FIGURE 2 shows an enlarged cross sectional view (not to scale) of thedynode 9 of FIGURE 1. The dynode 9 consists of an annular supportingring 17 of monocrystalline gallium arsenide P type semiconductivematerial of relatively low resistivity. Disposed on the supporting ring17 is a monocrystalline epitaxially-grown film 10 of a wide bandgapsemiconductor material such as gallium phosphide. The supporting ring 17may typicalls have a thickness x on the order of 5 mils, while thethickness of the epitaxial semiconductive film 10 may be on the order of2000 Angstroms.

Disposed on one surface of the semiconductive layer 10 is amonomolecular layer 11 of an electropositive work function reducingmaterial such as cesium. The layer 11 need not necessarily bemonomolecular, but should preferably have a thickness not exceeding avalue on the order of a few atomic diameters of the electropositivematerial.

Disposed on the upper surface of the ring 17 is a conductive layer 18comprising an acceptor material such as indium, which forms an ohmiccontact 'with the P type semiconductor material 17. The indium layer 18is preferably alloyed to the adjacent gallium arsenide layer 17.

While we have shown the supporting ring 17 as comprising monocrystallinegallium arsenide, this material is employed only for convenience ingrowing the epitaxial film 10, as it is extremely difiicult to grow anepitaxial layer on a non-crystalline substrate.

While we prefer to utilize a semiconductive film 10 of monocrystallinematerial, the film 10 may alternatively comprise polycrystallinesemiconductor material if the grain diameter of the polycrystallinematerial is on the same order as the thickness of the semiconductivefilm. Where polycrystalline material is employed, the supporting layer17 may comprise a suitable metal or other conductive material.

Electrical contact to the dynode '9 may be made by soldering orotherwise bonding a terminal lead to the indium layer 18; the terminallead may also serve to mechanically support the dynode within the shell2.

The semiconductive film 10 should comprise P type material which isheavily doped with acceptor impurities to provide a hole concentrationon the order of 10 to IO /cm. We prefer to employ beryllium as theacceptor material, although other suitable acceptors such as zinc ormagnesium could be utilized.

The manufacture of the dynode 9 is initiated by providing amonocrystalline Wafer of P type gallium arsenide of relatively low (onthe order of .01 to 1.0 ohm centimeters) resistivity. The galliumphosphide monocrystalline film 10 is epitaxially grown on the galliumarsenide wafer by the vapor phase reaction of gallium subchloride andphosphine according to Equation 1. Hydrogen is employed as the carriergas for the reactants.

GaCl+PH GaP+HCl+H 1) A small amount of beryllium chloride (BeCl is mixedwith the reactant gases to provide acceptor doping of the epitaxial film10. The beryllium chloride concentration is chosen so as to provide anet hole concentration in the film 10 on the order of 10 to 10 /cm.

The epitaxial deposition of the film 10 is continued until a thicknesson the order of 2000 Angstroms is reached, at which time the depositionprocess is terminated.

After cleaning the exposed surfaces of the gallium arsenide-galliumphosphide laminate, a suitable masking material is applied to theexposed surface of the gallium phosphide film 10 and to an annularperipheral portion of the exposed surface of the gallium arsensidewafer,

and the laminate is immersed in the following etching solution:

5 parts (by volume) concentrated sulphuric acid 1 part (by volume) of a30% volumetric solution of hydrogen peroxide in water 1 part (by volume)water This solution rapidly etches the exposed central portion of thegallium arsenide wafer. Since the gallium phosphide film is relativelyinsensitive to this etching solution, the etching process virtuallyhalts when the central portion of the gallium arsenide wafer has beenremoved.

After removing the masking material and washing the etched laminate, athin ring 18 of indium is placed upon and alloyed to the exposed surfaceof the remaining gallium arsenide ring 17.

One surface of the gallium phosphide film 10 is next carefully cleanedand heat treated to remove undesired contaminants, after which amonomolecular layer 11 of cesium is evaporated onto the cleaned andtreated surface. The resultant structure is that shown in FIGURE 2.

We have observed that when the gallium phosphide material of the film 10is bombarded with electrons having energies on the order of 200 electronvolts, a secondary emission ratio on the order of 100:1 can be obtained.That is, for each primary electron striking the surface 8 of the film10, an average of 100 or more secondary electrons are emitted from thelow work function opposite surface 12.

Since the film 10 comprises monocrystalline material (or polycrystallinematerial of proper grain size), electrons in the conduction band of thesemiconductor material can travel relatively long distances beforerecombining with holes in the material. The average distance which anelectron will travel in the film 10 before recombining is given by thediffusion length L defined as where L =diffusion length for electrons D=diffusion constant for electrons m=electron lifetime.

The diffusion constant D may be obtained from the Einstein relationship,

where ,u.=electron mobility e-=electronic charge k=Boltzmann constantT=absolute temperature.

Combining Equations 2 and 3, we have L .t 1- kT/e 4 The thickness of theepitaxial secondary emission semiconductive film 10 should besufficiently great so as to absorb substantially all incident primaryelectrons, but not so great that the secondary electrons produced cannotdiffuse through the semiconductive film to reach and be emitted from thesurface of the low work function electropositive layer 11. Thus, thethickness of the film 10 should not exceed a few times (usually notexceeding three times) the above-defined diffusion length L Preferably,the thickness of the film 10 should be on the order of the diffusionlength provided this does not conflict with the requirement that thefilm be sufficiently thick to absorb substantially all incident primaryelectrons. For the 2000 Angstrom gallium phosphide film of our example,the diffusion length L is on the order of 2000 to 3000 Angstroms at roomtemperature.

The theoretical basis for the electron emitting properties of the dynode9 will best be understood by reference to FIGURE 3.

FIGURE 3A shows a partial cross sectional view of the dynode 9 takenalong the cutting plane A-A' of FIGURE 2. Vertically aligned with FIGURE3A, FIGURE 3B shows an energy level diagram for the dynode structure. Itis seen from the diagram that the semiconductive film 10 has beendegeneratively doped with acceptor impurities so that the Fermi level inthe semiconductive film lies slightly below the top of the valence band.In practice, practical doping concentrations result in a Fermi level soclose to the top of the valence band that the difference between theenergy gap (between the top of the valence band and the bottom of theconduction band) and the energy differential AE (between the Fermi leveland the bottom of the conduction band) is negligible.

The electropositive metallic layer 11 constrains the Fermi level tosubstantially coincide with the bottom of the conduction band at theemitting surface 12. Assuming the dynode to be in thermal equilibrium,and the effect of any externally-applied electric field across thecomposite consisting of film 10 and layer 11 (due to the potentialdifferences V through V to be negligible, the Fermi level will besubstantially constant throughout the film 10 and layer 12.

Since the energy gap is substantially constant throughout the structure,the energy bands necessarily bend in the immediatevicinity of theemitting surface 12, as is seen in FIGURE 3B. The band bending extends adistance d into the film 10 which is small compared to the totalthickness D of the film. The actual value of the distance d dependsprimarily upon the energy gap and the impurity doping level within thesemiconductive film 10.

The energy level diagram indicates that within a very small distance 6inward from the electropositive layer 11, the Fermi level lies closer tothe bottom of the conduction band than to the top of the valence band,while in the bulk of the film 10 the opposite is true. It is thereforeevi dent that the bulk of the film exhibits P type conductivity, whilethe extremely thin (inversion) region within a distance 6 from theelectropositive layer 11 behaves as though it were of N typeconductivity.

If the electropositive layer 11 on the film 10 has a work function lessthan the energy differential AE (or in our example the substantiallyequivalent energy gap), the vacuum energy level at the emitting surface12 (corresponding to the energy at electron must have to escape from thesurface) will lie below the bottom of the conduction band in the bulk ofthe semiconductive film 10.

Due to this height of the conduction band bottom above the vacuum energylevel, electrons in the conduction band of the semiconductive film 10(secondary electrons in the conduction band are produced as a result ofcollisions with incident primary electrons) may diffuse toward theemitting surface 12 while retaining a residual energy at the emittingsurface which is greater than the vacuum energy level. The net result isthat electron emission can take place from the surface 12 without thenecessity for applying any external force to overcome the surface workfunction and the dynode behaves as though it had a negative effectiveelectron aflinity.

The high doping levels employed in the semiconductive film 10 accordingto our preferred embodiment insure that the distance d in which bandbending occurs is extremely small (on the order of 30 to angstroms), sothat energy loss of electrons diffusing toward the emitting surface 12is minimized.

For the dynode of our preferred embodiment, the gallium phosphidematerial employed has an energy gap on the order of 2.3 electron voltswhile the work function exhibited by the cesium-coated emitting surfaceis on the order of 1.3 electron volts, resulting in a negative effectiveelectron affinity of approximately 1.0 electron volt.

We claim:

1. A secondary emission device, comprising:

a source of free electrons;

at least one dynode including a film of semiconductor material havingfirst and second opposed major surfaces,

said material having spaced valence and conduction energy bands with agiven energ gap therebetween, said material having a net excessconcentration of acceptor impurities such that the Fermi level in saidmaterial lies relatively proximate to the uppermost energy level of thevalence band and relatively remote from the lowermost energy level ofthe conduction band, said dynode including a thin layer comprising anelectropositive Work function reducing material on said second surface,said layer having a thickness not exceeding a value on the order of afew atomic diameters of said electropositive material, saidelectropositive layer reducing the work function of said second surfaceso that the vacuum energy level at said second surface lies below thelowermost energy level of the conduction band in the bulk of said film;means for accelerating said free electrons to impinge upon said firstsurface with a kinetic energy sufficient to produce secondary electronsin the conduction band of said material; the thickness of said filmbeing sufficiently great so that said impinging electrons are absorbedby the film, said film thickness not exceeding a value on the order of afew times the diffusion length for electrons in said material,

whereby said secondary electrons may diffuse toward and be emitted fromsaid second surface; and means for utilizing the secondary electronsemitted from said second surface. 2. A secondary emission deviceaccording to claim 1, wherein said impurity concentration is sufficientto provide a hole concentration on the order of 10 to 10 /cm. in saidmaterial.

3. A secondary emission device according to claim 1, wherein said filmis monocrystalline.

4. A secondary emission device according to claim 1, wherein said filmis polycrystalline with an average grain diameter on the order of thethickness of said film.

5. A secondary emission device according to claim 1, wherein saidmaterial comprises gallium phosphide.

6. A secondary emission device according to claim 5, wherein saidimpurity comprises beryllium.

7. A secondary emission device according to claim 6, wherein said filmthickness is on the order of 2000 angstroms.

8. A secondary emission device according to claim 5, further includingfilm supporting means comprising an annular ring of monocrystallinegallium arsenide, said film being epitaxially grown on said supportingmeans.

9. A secondary emission device according to claim 7, wherein the kineticenergy of said impinging free electrons is on the order of 2000 electronvolts.

References Cited UNITED STATES PATENTS 2,898,499 8/1959 Steinglass et al3l3105 3,150,282 9/1964 Geppert 3l3346 3,214,629 10/1965 Apker 313-3463,334,248 8/1967 Stratton 313346 3,364,367 1/1968 Brody 3l3103 RALPH G.NILSON, Primary Examiner M. ABRAMSON, Assistant Examiner U.S. Cl. X.R.

Egg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent3.471121% Dated November ll 1069 Inventor(s)Ral h E. Simon, Brown F.Williams 6 Ralph Wasserman It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 4, line 11 "typicalls" should read typically Column 5, line 24"200" should read 2000 Column 5, line 38, Equation (2) should readColumn 5, line 49 "u" should read "u SIGNED AND S E A 1 ED MAY 2 e 1970(SEAL) Attest:

Edward M. Fletcher, Jr. WILLIAM E. m Attesting O f Gommissioner ofPatents

